The present invention relates to automatic chroma control circuitry for a color television receiver in which a reference signal containing unique hue and saturation information is relied upon for automatically establishing correct hue and saturation in the displayed image.
The invention relates to control circuits of the type set forth in U.S. Pat. No. 3,673,320 issued to Carnt et al., June 27, 1972 and entitled Television Apparatus Responsive to a Transmitted Color Reference Signal. The Carnt et al. patent shows a possible form of a VIR signal and its use in a color television receiver to automatically obtain proper hue and saturation setting in the image displayed. The Carnet et al. patent is directed generally to the use of a VIR signal for automatic chroma control. However, assumptions are made in the teachings of this patent that leave problems to be solved in order to implement the practical design of a receiver. First, it is assumed that a suitable reference corresponding to the zero level of the color difference detector is applied to the hue control circuit and a suitable reference corresponding to the zero level of the blue amplifier is applied to the saturation control circuit, although no such references are shown or described. Secondly, it is assumed that the circuitry responsive to the VIR signal which establishes the zero hue information and the zero saturation information does not drift. Thirdly, it is assumed that whatever reference level is used is free from drift.
It is known that feedback type control circuits require a suitable reference signal. It is also known that the circuit elements employed in today's television receivers are subject to drift due to temperature and age. In addition, if the reference employed in the hue and saturation control circuits drifts unacceptable color rendition will result.
Another patent which deals with the use of a VIR signal for automatic hue control in a color television receiver is U.S. Pat. No. 3,780,218, issued to John Rennick, Dec. 18, 1973 and entitled Circuit for Establishing Correct Hue Setting in Color Television Using VIR Signal. Unlike the Carnt et al. patent Rennick shows the use of a reference in his hue control circuit and does show the VIR signal currently being considered for adoption by the Federal Communication Commission. Rennick like Carnt et al., however, fails to acknowledge the drift problems with present day receiver circuits components and fails to utilize the reference contained in the VIR signal to correct for such drift problems.
Both the Carnt et al. patent and the Rennick patent deal with the proposition that when the phase of the chrominance reference portion of the VIR signal is the phase of one transmitted color difference signal then the other color difference signal is zero. Thus, if the phase of the chrominance reference is -(B-Y) then the R--Y detector output should be zero, i.e., the detector should have the same output that it has when no chroma is transmitted. This unique characteristic permits the use of a feedback control circuit to set the hue of the receiver by automatically adjusting the tint control so that the R-Y detector indication is zero. Carnt et al. further teach that specific luminance to chrominance proportioning of the chrominance reference creates the condition where one of the color signals is zero. Specifically, if the phase of the chrominance reference is -(B-Y) and the ratio of luminance to chrominance amplitudes is 2.03 then the blue signal should be detected as zero, i.e., the detector should have the same output level that it has during black level transmission. Accordingly, proper setting of the saturation of the displayed image for each luminance setting is realized by use of an automatic control circuit to adjust the chroma gain until the blue signal indication is zero.
The aforementioned drift problems with the prior art in the practical utilization of a VIR signal for automatic chroma control are solved by the present invention. First, it was discovered that the VIR signal proposed to the Federal Communication Commission in Docket 19907 and presently being considered for adoption is much more suitable for practical utilization in a receiver than the signal format shown in the aforementioned Carnt et al. patent. The signal format being considered is identical to that shown in the Rennick patent and unlike the Carnt et al VIR signal format includes a black reference signal level which, according to the present invention provides the opportunity to determine and update the hue null and saturation null references. The present invention contemplates double interrogation of the VIR signal, a first time during the black level reference interval to obtain a measure of a zero reference for the two automatic control and a second time during the chroma reference interval to correct the hue and saturation setting of the receiver by a comparison of the hue and saturation indications from the chroma reference with the measured zero reference. In this manner, a continually updated reference for correct hue and saturation is made readily available. Drift problems not considered by either Carnt et al. or Rennick, namely drift in the R-Y detector output or drift in the loop reference, are overcome by the present invention by the periodic updating of the loop reference.
It is accordingly an object of the present invention to provide a practical automatic chroma control circuit for a color television receiver which is insensitive to drift problems.
Another object of the present invention is to employ double interrogation of the VIR signal to realize properly referenced automatic hue and saturation control circuits for a color television receiver.
These and other objects are generally realized in the preferred embodiment by the provision in the hue control circuit (the saturation control circuit may be substantially identical to the hue control circuit of a first switch operative during the black level reference portion of the VIR signal to apply the output of the R-Y detector to a storage circuit. The R-Y output present during this interval is stored as the hue null reference. This hue null reference is applied to one input terminal of a differential amplifier, the second input terminal of which is coupled directly to receiver the R-Y output. Thus, when the feedback control circuit is closed by a second switch during the chroma reference portion of the VIR signal, any differential existing at the input of the differential amplifier is reduced to zero by automatic control of the phase of the regenerated subcarrier. In this manner the hue setting of the receiver is updated during each receipt of the VIR signal.